param: abstract 'struct command' so param doesn't need to access it.

I want to use param functions in plugins, and they don't have struct
command.

I had to use a special arg to param() for check to flag it as allowing
extra parameters, rather than adding a one-use accessor.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/jsonrpc.h

@@ -38,8 +38,6 @@ struct command {
 	/* This is created if mode is CMD_USAGE */
 	const char *usage;
 	bool *ok;
-	/* Do not report unused parameters as errors (default false). */
-	bool allow_unused;
 	/* Have we started a json stream already?  For debugging. */
 	bool have_json_stream;
 };